1. Field of the Invention
The present invention concerns a gas laser device that emits ultraviolet rays, especially a gas laser device that emits ultraviolet rays, such as an excimer laser device having high oscillation efficiency.
2. Description of Related Art
Higher resolution is demanded of projection exposure equipment as the miniaturization and integration of semiconductor integrated circuits rise. Consequently, the wavelength of exposure light emitted from exposure light sources is becoming shorter, and gas laser devices that emit ultraviolet rays, such as ArF excimer laser devices or fluorine laser devices, would be viable candidates for the next generation of semiconductor exposure light sources.
Mixed gas comprising fluorine (F2) gas, argon (Ar) and noble gases, such as neon (Ne), as a buffer gas in an ArF excimer laser device, or mixed gas comprising fluorine (F2) gas and noble gases, such as helium (He) as a buffer gas, in a fluorine laser device would be sealed within a laser chamber at a pressure of several 100 kPa, and a pair of main discharge electrodes would be mounted facing each other with a prescribed separation. Laser gas, the laser medium, would be excited within the laser chamber by generating discharge at the main discharge electrodes.
Uniform discharge must be generated between the main discharge electrodes to efficiently generate laser light; but, the laser gas that is present in the discharge space between the main discharge electrodes is commonly subjected to preionization before the main discharge commences in order to generate a uniform discharge in a high-pressure gas atmosphere of several 100 kPa.
One means of generating the preionization would be the preionization method in which two electrodes are disposed facing each other with a dielectric interposed between them. Examples of such preionization units are presented in Japanese Kokai Publication Hei-5-327070, U.S. Pat. No. 2,794,792, Japanese Kokai Publication Hei-10-242553, Japanese patent publication No. 8-502145 and U.S. Pat. No. 5,337,330. All of the preionization units noted are structured with a first electrode (hereinafter abbreviated outer electrode) in contact with the outer surface of a tube formed from a dielectric and a second electrode (hereinafter abbreviated inner electrode) that is inserted within the tube. Corona discharge is created between the outer electrode and the dielectric tube by generating a potential difference between the outer electrode and the inner electrode, and laser gas that is present in the discharge space between the main discharge electrodes is subjected to preionization by ultraviolet light that is generated at this time. There are also cases, in addition to the preionization units in which the dielectric tube and the outer electrode are in proximity without making contact, as well as cases in which the outer electrode is covered by a dielectric substance.
FIG. 5 is a block diagram of an excitation circuit of a gas laser device that emits ultraviolet rays (hereinafter abbreviated gas laser device) using the preionization method. This excitation circuit has a circuit structure termed a charge transfer circuit that uses a solid state switch SW such as a JGBT. In a simple explanation of the operation following this circuit diagram, charge from a high voltage power source HV is held in capacitor C1 when switch SW is opened. When switch SW is closed while the charge is held in capacitor C1, the charge of capacitor C1 transfers to capacitor C2. The charge that had transferred to capacitor C2 is then transferred to peaking capacitor C3 via non-linear inductance Lm termed a saturable inductance or a magnetic switch. The pulse amplitude of the voltage that is applied through the action of magnetic switch Lm is compressed. The operation of magnetic switch Lm is that the inductance increases while the charge of capacitor C1 is transferred to capacitor C2, and the inductance rapidly decreases upon saturation when the magnetic flux density has increased, thereby efficiently transferring the charge of capacitor C2 to peaking capacitor C3. Pulse discharge develops between the facing main discharge electrodes 3, 4 within laser chamber 1 when the voltage of peaking capacitor C3 has risen and has reached the discharge breakdown voltage. Laser gas is then excited. Specifically, current flows through the discharge circuit loop shown by the thick lines in FIG. 5 as a result of this discharge.
A differential voltage circuit comprising capacitors C11, C12 and the inductances L0 are connected in parallel to charge electrodes 3, 4. The pulse voltage applied between the main discharge electrodes 3, 4 is divided, as shown in FIG. 6, and is lowered to a range of 25% to 75% thereof, after which voltage is applied in order to attain corona discharge between the outer electrodes 9 and inner electrodes 7 within corona preionization units 15 that are disposed near the upstream side and downstream side of the main discharge space between main discharge electrodes 3, 4. The optimum values of the differential voltage ratio, the capacitance of capacitors C11, C12, and inductance L0 are selected, the time constant is set to the desired value, and the timing of corona preliminary discharge versus the main discharge is adjusted. The composite capacitance of this differential voltage circuit is adjusted to a level under 10% of peaking capacitor C3.
Incidentally, the laser oscillation efficiency is known to be enhanced as the inductance created by the discharge circuit loop falls (Mitsuo Maeda ed. xe2x80x9cExcimer laserxe2x80x9d pp. 64-65, Gakkai Shuppan Center Inc. first edition Aug. 20, 1983)
FIG. 6 shows a block diagram of an actual discharge circuit loop mentioned above. FIG. 6 is a cross-sectional view of the principal parts of a gas laser device perpendicular to the direction of laser oscillation. Those constituent elements given the same notation in FIG. 6 as in FIG. 5 correspond to the constituent elements shown in FIG. 5.
In a simple explanation, insulation base 21 is inserted in an airtight manner on the upper wall of laser chamber 1 so as to lie along the longitudinal direction of the discharge space. The other main discharge electrode 3 (for example, a cathode) is attached to the insulation base 21 centrally inside of laser chamber 1 and is connected to high voltage power source 10 via current induction unit 23 penetrating the insulation base 21. Here, high voltage power source 10 corresponds to the circuit section containing non-linear inductance Lm on the left side of the peaking capacitor C3 in FIG. 5. A pair of conduction units 25 are attached roughly parallel to insulation base 21 so as to lie on both sides of main discharge electrode 3 within laser chamber 1. Electroconductive base 26 is extended across the ends of conduction unit 25 and one of the main discharge electrodes 4 (for example, the anode) is attached at the center opposite main discharge electrode 3 at the top in the center. Peaking capacitors C3, comprising a plurality of capacitors connected in parallel, are connected to both sides of current induction unit 23 outside of laser chamber 1. Peaking capacitors C3 are connected to conduction unit 25 via the current induction unit 24 that pierces insulation base 21. Furthermore, preionization unit 15, in which outer electrode 9 and inner electrode 7 are disposed facing each other with interposed dielectric tube 8, is disposed at the view position of the main discharge space, between the main discharge electrodes 3, 4, upstream and downstream of the laser gas stream 2 (denoted by arrows above electroconductive base 26). Outer electrode 9 is connected directly to electroconductive base 26 while inner electrode 7 is connected between capacitor C11 and C12 of high voltage power source 10 via a terminal that is not illustrated.
The section enclosed by broken lines in the structure shown in FIG. 6 is the discharge circuit loop explained in FIG. 5. It comprises the current induction unit 23 that pierces insulation base 21, main discharge electrode 3 connected to current induction unit 23, main discharge electrode 4, electroconductive base 26 in which main discharge electrode 4 is installed, conduction unit 25 connected to electroconductive base 26, current induction unit 24 that is connected to conduction unit 25 and which pierces insulation base 21, and peaking capacitor C3 to which current induction unit 24 and current induction unit 23 are connected.
As mentioned above, the laser oscillation efficiency is enhanced as the inductance created by the discharge circuit loop falls. Since the inductance is proportional to the cross-sectional area of the discharge circuit loop (area of the cross section in FIG. 6), it must be structured so as to minimize the cross-sectional area. Specifically, this must be structured so that the cross-sectional area of the space enclosed by the broken line in FIG. 6 that includes current induction unit 23, main discharge electrode 3, main discharge electrode 4, electroconductive base 26, conduction unit 25, current induction unit 24, peaking capacitor C3 is small.
However, the potential difference of current induction unit 23, main discharge electrode 3, peaking capacitor C3 from laser chamber 1 that is usually grounded is great, at 20 to 30 kV, which brings about dielectric breakdown if the separation is too close. Accordingly, the size of insulation base 21 cannot be too small.
Furthermore, the separation of the main discharge electrode 3 and the main discharge electrode 4 determines the magnitude of the laser light that is emitted, but the size of the laser light is restricted to a certain extent as a function of the application. For example, the separation would be 15 to 18 mm for an ArF excimer laser used in semiconductor exposure, and it cannot be made too short.
In addition, the size of the electroconductive base 26 cannot be too small since preionization units 15 are disposed on both sides of the main discharge electrode 4.
Furthermore, the cross-sectional area of the discharge circuit loop can be reduced as the position of the conduction unit 25 that links the electroconductive base 26 with the current induction unit 24 approaches preionization unit 15. However, conduction unit 25 begins to act like outer electrode 9 as the conduction unit 25 approaches the preionization unit 15 since conduction unit 25 has the same potential as that of the outer electrode 9 that forms preionization unit 15. When that happens, corona discharge takes place on even the side opposite from that of the discharge space between main discharge electrodes 3, 4. The ultraviolet rays that are created due to this corona discharge no longer reach the discharge space, and thus, does not contribute to preionization of laser gas present in the discharge space. Specifically, the energy supplied for corona discharge that occurs between dielectric tube 8 and outer electrode 9 decreases due to the excess corona discharge, and that leads to the potential for inadequate preionization.
Furthermore, in the case of the UV arc preionization method since discharge breakdown develops between the high voltage side of the preionization electrode and a conduction member when the conduction member is positioned outside of a pair of electrodes for preionization (opposite side from the electrode) and is brought too close to the electrodes for preionization, they should not be brought too close together as stated in Japanese Kokai Publication Hei-3-145170 and Applied Physics B, Vol. 63, pp. 1-7. Consequently, the cross-sectional area of the discharge circuit loop cannot be made too small.
The inventors have determined that the inductance created by the discharge circuit loop shown in FIG. 6 would be a minimum of 10 nH in the case of a conventional excimer laser device.
The present invention was devised to resolve the problems associated with conventional technology. The purposes are to reduce the cross-sectional area of a discharge circuit loop in an excitation circuit of a gas laser device that emits ultraviolet rays, to reduce the inductance and to enhance such characteristics as the laser oscillation efficiency.
A gas laser device that emits ultraviolet rays and attains the purposes is provided with a laser chamber in which laser gas is sealed and which has a circulation means that circulates this laser gas within the laser chamber, a pair of main discharge electrodes disposed at a prescribed separation within the laser chamber, a discharge circuit comprising peaking capacitors that are connected in parallel with this pair of main discharge electrodes, and a preionization means in which a first electrode and a second electrode are disposed facing each other with a dielectric interposed between them, wherein this preionization means is disposed near both sides of one of the main discharge electrodes so as to run alongside thereof, wherein one of the main discharge electrodes and the peaking capacitors are connected via a conduction member that passes between one of the main discharge electrodes and the preionization means.
In this case, the conduction member comprises a conductor plate with an aperture opened within it, the aperture transiting laser gas that passes through the main discharge space between the main discharge electrodes and being disposed so that ultraviolet rays from the preionization means reach the main discharge space.
Furthermore, the preionization means is structured from a second electrode covered by a dielectric substance and a first electrode that makes contact with the outer surface of the dielectric substance about the periphery of the second electrode, and the conduction member and the first electrode should be integrated.
The characteristics of the gas laser device that emits ultraviolet rays, such as the laser oscillation efficiency, can be enhanced since the cross-sectional area of the discharge circuit loop in the excitation circuit can be reduced and the inductance of the discharge circuit loop can be reduced because one of the main discharge electrodes and the peaking capacitor are connected by a conduction member that passes between one of the main discharge electrodes and the preionization means in the present invention.